Marker (telecommunications)

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For other meanings, see the disambiguation page Marker

A marker is a type of special purpose control system that was used in electromechanical telephone central office switches. Central office switches are the large devices that telephone companies use to make the connections that support telephone calls. So the switch will be used to make the voice connections between users, and to make connections to other equipment such as that used to detect the tones in DTMF (commonly called TouchTone) dialing.

Markers were sometimes referred to as special purpose computers but, lacking stored program control, they were not computers according to the understanding of the middle 20th Century. After unfruitful German efforts in the 1920s, they were successfully developed at Bell Labs in the 1930s to support the then new generation of crossbar switches which were replacing the step by step switches or Strowger switches of the first generation of automatic switching.

Markers were built from relays (wire spring relays and other kinds). Different types of markers performed various specialized hard-wired operations. For example, a dial tone marker could select one of a number of shared digit receivers and connect it to a subscriber who wished to make a telephone call. The digit receiver would collect the digits of the call and make them available to other markers which would use them for routing purposes. The line link marker would mark a proper path of idle links for the call to make through the mechanical voice switching matrix.

Markers were used in the design of switches from the 1930s until the late 1960s when they were replaced with software controlled electronic computers of modern design.

The term marker came from their use to mark a path of links through the switching fabric. During the middle 20th Century they acquired other functions, including outgoing digit translation and enforcement of different policies upon different classes of service in the provision of features to customers. This practice evolved into Customer Groups, allowing the addition of Centrex features to 5XB. These were the most complex markers made, and were abandoned in the 1970s when Stored Program Control became mature.